The senile plaque and congophilic angiopathy are abnormal extracellular structures found in abundance in brain of patients with Alzheimer's disease. The biochemical composition of these structures has been extensively studied to better understand their possible role in the pathogenesis of this dementing disease. The mature senile plaque is a complex structure, consisting of a central core of amyloid fibrils surrounded by dystrophic neurites, axonal terminals and dendrites, microglia and fibrous astrocytes. See D. Selkoe Neuron 6, 487-498 (1991). The amyloid core of the senile plaque surrounding blood vessels, producing the congophilic angiopathy, is a peptide of 39 to 43 amino acids termed the .beta.-Amyloid (.beta.A) peptide. G. Glennet and C. Wong, Biochem. Bioshys. Res. Comm. 120, 885-890 (1984). .beta.A peptide is found in brain in Alzheimer's disease, Down's syndrome, hereditary cerebral hemorrhage of the Dutch type, and in old age. K. Kosik, Science 256, 780-783 (1992). .beta.A is produced by abnormal proteolytic processing of a larger protein, the amyloid precursor protein (APP). See K. Bayreuther and C. Masters, Brain Path. 1, 241-251 (1991).
The senile plaque and congophilic angiopathy contain proteins in addition to .beta.A peptide. APP itself, among others, has been identified in the senile plaque by histochemical studies employing antibodies recognizing either the amino- and carboxy- termini of the precursor protein. See, e.g., F. Tagliavine et al., Neurosci. Lett. 128,117-120 (1991); C. Joachim et al., Amer. Jour. Path. 138, 373-384 (1991); The mechanisms by which these proteins aggregate in the extracellular space to associate with the senile plaque and congophilic angiopathy are not known.
Apolipoprotein E (ApoE) performs various functions as a protein constituent of plasma lipoproteins, including its' role in cholesterol metabolism. It was first identified as a constituent of liver-synthesized very low density lipoproteins which function in the transport of triglycerides from the liver to peripheral tissues. There are three major isoforms of ApoE, referred to as ApoE2, ApoE3 and ApoE4 which are products of three alleles at a single gene locus. Three homozygous phenotypes (Apo-E2/2, E3/3, and E4/4) and three heterozygous phenotypes (ApoE3/2, E4/3 and E4/2) arise from the expression of any two of the three alleles. The most common phenotype is ApoE3/3 and the most common allele is E3. See Mahley, R. W., Science 240:622-630 (1988).
The amino acid sequences of the three types differ only slightly. ApoE4 differs from ApoE3 in that in ApoE4 arginine is substituted for the normally occurring cysteine at amino acid residue 112. The most common form of ApoE2 differs from ApoE3 at residue 158, where cysteine is substituted for the normally occurring arginine. See Mahley, Science, supra.
While there has been considerable research into the mechanisms underlying Alzheimer's disease, there continues to be an ongoing need for new ways to investigate and combat this disorder.